Antidiuretic Effect of Eel ANP Infused at Physiological Doses in Conscious, Seawater-Adapted Eels, Anguilla japonica

Atrial natriuretic peptide (ANP) is known as a potent natriuretic/diuretic hormone in vertebrates. However, eel ANP infused at doses that did not alter arterial blood pressure (0.3-3.0 pmol/kg/min) decreased urine volume and increased urinary Na concentration in seawater (SW)-adapted eels but not in freshwater (FW)-adapted eels. The renal effects were dose-dependent and disappeared after infusate was switched back to a vehicle (0.9% NaCl). Urinary Na excretion (volume x Na concentration) did not change during ANP infusion. ANP infusion increased plasma ANP concentration, but the increase at the highest dose was still within those observed endogenously after injection of hypertonic saline. Urinary Mg and Ca concentrations increased during ANP infusion in SW eels, but urinary Ca excretion decreased in FW eels. Plasma Na concentration profoundly decreased during ANP infusion only in SW eels, suggesting that ANP stimulates Na extrusion via non-renal routes. These results indicate that ANP is a hormone which specifically extrudes Na ions and thereby promotes SW adaptation in the eel. This is in sharp contrast with mammals where ANP is a volume regulating hormone that extrudes both Na and water.